Laboratory studies and modelling of mesospheric iron chemistry

Both neutral and ionized iron species occur in the mesosphere and lower thermosphere as a result of meteoric ablation. This paper examines two phenomena that are of current interest: the formation of sporadic neutral Fe layers from sporadic E layers; and the chemical amplification that can occur when the atomic Fe layer is perturbed by atmospheric gravity waves. Understanding these processes requires the development of a detailed chemical model, where the rate coefficients of the individual reactions have been measured in the laboratory. A novel experimental system for studying the reactions of Fe-containing species is described. Pulses of atomic Fe and Fe + were produced in the upstream section of a fast flow tube by the pulsed laser ablation of a pure Fe rod, and detected at the downstream end by laser induced fluorescence and mass spectrometry, respectively. This apparatus was used to study the reactions of neutral and ionic Fe-containing molecules with atomic O and H. These reactions influence the appearance of the Fe layer by governing the rates at which iron is converted from reservoir species (e.g. FeOH, Fe +) back to Fe. The new model of the Fe layer that results from this experimental work is then used to show that sporadic Fe layers can form from descending sporadic E layers, and that the Fe layer should exhibit significant chemical amplification below 90 km when perturbed by gravity waves.